Estimate first: a mental math trick
When to use it: Before a long calculation, and to check an answer.
Worked example
- Round each number to its first digit.
- Do the easy calculation.
- Your exact answer should land near it. If it is 10 times off, a zero or a decimal point slipped.
Round each number to its first digit, then multiply: 412 × 38
- 412 rounds to 400, and 38 rounds to 40.
- 400 × 40 = 16,000.
- The exact answer, 15,656, is close.
Answer: 16,000
Try three
-
Round each number to its first digit, then multiply: 744 × 82
Show the answer
56,000
-
Round each number to its first digit, then multiply: 291 × 67
Show the answer
21,000
-
Round each number to its first digit, then multiply: 392 × 79
Show the answer
32,000